Extremely short duration optical pulses are important for high speed signal processing and communications. Optical pulses in this category are commonly called ultrashort optical pulses and have durations in the picosecond and sub-picosecond range. Ultrashort optical pulses have been obtained by techniques such as passive modelocking wherein a nonlinear element is placed either within the lasing cavity or within a cavity external to the lasing cavity. One such nonliner element is a saturable absorber.
Saturable absorbers are materials which display a change in opacity related to the incident radiation intensity at a particular wavelength. Solids, liquids, and gasses may be used as saturable absorbers based upon the chosen wavelength of operation. The saturable absorber acts as a shutter. It absorber all weak radiation incident upon it. As the intensity of the incident radiation reaches a sufficiently high level called the saturation intensity of the saturable absorber, the incident radiation is permitted to pass through the absorber. In general, the attenuation of the incident radiation caused by the saturable absorber is relatively low because the absorber is saturated into a transparent state at the wavelength for the incident radiation.
In practice, the nonlinearity and loss of a saturable absorber is determined effectively by the cross-section or thickness of the saturable absorber material traversed by the incident radiation. For thicker materials, a larger nonlinearity (saturation intensity) is achieved together with a larger loss for the saturable absorber; for thinner materials, a smaller nonlinearity is achieved together with a smaller amount of loss. Whether used as an intra-cavity or external cavity element, the saturable absorber exhibits a nonlinearity and loss which are intimately coupled. This limits the design flexibility for known saturable absorbers because there is no available technique for designing the nonlinearity or saturation intensity independent from the loss of the saturable absorber.